How to determine crane bottlenecks in the EAF charging bay
This is for anyone planning to add an EAF to an existing BOF melt shop. To verify the crane does not become a bottleneck — potentially slowing down the EAF — follow these steps.
Step-by-step method
1. List all crane activities. Write down every crane task and the frequency it occurs. Don’t forget transports to bucket stand-by positions. With rail-based scrap buckets, you need to quickly free the current bucket and place a new one to allow efficient loading of the next bucket.
2. Measure travel distances. For each task, measure from pick-up position to set-down location based on your drawings. Consider parallel X and Y movements, and make the Z movement sequential.
3. Calculate task durations. Use average crane velocities of bridge, trolley, and hook. Add 15 seconds for hooking or unhooking scrap buckets. For other tasks, adjust based on your experience.
4. Stop. Avoid the temptation to simply sum all task durations to calculate crane utilization. This is likely to mislead your judgment.
5. Draw a Gantt chart. Place all crane activities and EAF heats in a Gantt chart using Excel. Each crane and EAF gets its own row. Use narrow columns for 1-minute time steps. Draw rectangular shapes for crane activities and snap to the grid.
6. Start with EAF heats. They are the pace-setter and the simplest to manage.
7. Add scrap bucket activities from your crane task list, timing them to the EAF charging times.
8. Backfill all remaining tasks.
9. Resolve conflicts. Things will get crowded. Adjust starting times by rearranging crane activities. If you need to delay the charging task, extend or move the EAF heat accordingly. Eventually you’ll arrive at a workable solution.
10. Validate with pen and paper. Draw your bay, EAFs, and all important positions on a sheet. Use pens as cranes and move them according to the Gantt chart. Identify any crane conflicts and extend task durations accordingly. Repeat until validated.
11. Done. You now have a feasible production and logistics plan for the charging bay.
What if it doesn’t work?
If the EAF will not run smoothly, go back to step 1 and work on eliminating tasks by altering your design concept:
- Automatic electrode tilting for deposit and hoisting stations
- Two hooks for electrode exchange or automatic electrode jointer
- Jib cranes for auxiliary tasks
Never consider only the base-load case where everything is perfect. Plan for the challenging situations — that’s where bottlenecks appear.
This approach is the second-best method available. The best method is a full simulation of the charging bay, where all crane movements, conflicts, and timing are resolved dynamically.